<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    *{margin: 0;padding: 0}
    img{
        width: 200px;padding: 20px;
    }
    #parent{
        margin-left: auto;margin-right: auto;
        position: relative;
        width: 240px;
        border: 1px solid #333;
    }
    #fade {
        width:100%;height: 100%;
        position: absolute;
        background: palevioletred;
        opacity: 0.3;
        left: 0;
        top: 0;
    }
    </style>
</head>
<body>
    <div id="parent">
        <img src="images/timg (2).jpg" alt="">
        <div id="fade"></div>
    </div>
    <script>
        var parent = document.getElementById("parent");
        var fade = document.getElementById("fade");
        var opacity = getComputedStyle(fade).opacity * 100;
        var timer;
        // 使用定时器做动画
        /*
        1.执行事件时首先清除定时器
        2.定时器中的内容,用if-else 
         */

        parent.onmouseover = function () {
            clearInterval(timer);
            timer = setInterval(function () {
                if (opacity > 70) {
                    clearInterval(timer)
                } else {
                    opacity += 2;
                    fade.style.opacity = opacity / 100;
                }

            }, 50)
        }
        parent.onmouseout = function () {
            clearInterval(timer);
            timer = setInterval(function () {
                if (opacity < 30) {
                    clearInterval(timer)
                } else {
                    opacity -= 2;
                    fade.style.opacity = opacity / 100;
                }
            }, 50)
        }
    </script>
</body>
</html>